Morphogenesis is characterized by the specialization of cells which, in response to various signals, coordinate growth, movement, and interactions such that these events give rise to a functional organism. One of the major problems in developmental biology is to identify those signals and determine their mechanism(s) of action. Many models attribute an inherent role to the cell surface, either invoking specific cohesion molecules or modifications of more general components, but limited progress has been made in identifying these structures. The ultimate goal of this research is to understand how specific cell-cell contact formation can influence the spacial and temporal organization of a metazoan. To approach this goal, we will undertake a study of a surface protein involved in cell cohesion in a model system, D. discoideum. The protein was identified by the ability of a monoclonal antibody to inhibit cell-cell contact formation at a specific developmental stage. We are now in a position to characterize the protein and the possible modifications which may affect its expression and function. Part of such studies involves purifying the protein to raise polyclonal sera and a panel of monoclonals directed against different moieties of the protein. Such sera will be invaluable in studies of the regulation of the protein at transcriptional and post-transcriptional levels as well as in disecting the mechanism(s) by which it functions to affect selective cell cohesion. For these and future studies involving, e.g., site-directed mutagenesis, a most valuable tool will be that provided by the mutants to be isolated which are defective in the protein. Continued investigations will lead to examining if this cohesion molecule also plays a role in the spacial and temporal organization of the developing organism.